1. Technical Field
The present disclosure relates to spindle motors and, more particularly, to a method and device for stopping a spindle motor.
2. Description of the Related Art
In the state of the art, it is known that the hard disks of computers and the like are provided with a spindle motor for rotating the hard disk itself and a voice coil motor for moving the reading and writing heads.
In the event of a sudden lack of supply voltage, a procedure is implemented to first park the reading and writing heads, and then to stop the spindle motor once the heads have reached the parked position.
In the absence of supply voltage, the spindle motor serves as a voltage generator, the voltage of which depends on its rotational speed and its electric constant.
FIG. 1 shows the block diagram of a typical power combination used in applications for hard disks. The power combination is used to properly drive the spindle motor 2 and the voice coil motor 3 by a single driving circuit 1, which properly drives the power stage 12 for the spindle motor 2 and the power stage 13 for the voice coil motor 3, as better shown in FIG. 2. The driving circuit 1 includes the driving circuit 10 of the power stage 12 of the spindle motor 2 and the driving circuit 11 of the power stage 13 of the voice coil motor 3. The power stage 12 includes three pairs of transistors M1-M2, M3-M4, M5-M6 with respective pairs of diodes D1-D2, D3-D4, D5-D6 connected in parallel with each other between supply voltage Vm and ground GND, whereas the power stage 13 includes two pairs of transistors M7-M8, M9-M10 with respective pairs of diodes D7-D8, D9-D10 connected in parallel with each other between a supply voltage Vm and ground GND. The spindle motor 2 is coupled to the shared terminals of the pairs of transistors M1-M2, M3-M4, M5-M6, whereas the voice coil motor is coupled to the shared terminals of the pairs of transistors M7-M8, M9-M10.
The external supply voltage VCV feeding the power part varies according to the type of application; in desktop applications (desktop PC) it is of 12V, whereas in mobile applications (laptop PC) it is of 5V.
The power combination, in addition to integrating the control of the two motors (spindle and voice coil motors), includes devices to implement other functions, i.e., voltage regulators and power monitor 4, serial port 5 and ISO-Fet 6.
The ISO-Fet 6 is an internal element of the power combination that serves to insulate the internal supply line Vm from the external supply line VCV if the latter were to fail.
The ISO-Fet 6 power up includes a transistor connected to the voltage VCV and controlled by signal P; said signal P is adapted to shut down the transistor of the ISO-Fet 6 when the voltage VCV is null, whereas it is adapted to keep it on when the voltage VCV is positive.
When the VCV fails, the backelectromotive force voltage of the rotating spindle motor, i.e., the BEMF (Backelectromotive Force), is rectified to keep the internal supply line Vm at a potential enough to supply the section of the voice coil motor 3 for parking the heads.
The rectification of the backelectromotive force of the rotating spindle motor may be carried out by means of one of the following procedures, i.e., a passive rectification, a synchronous rectification of the BEMF of the spindle motor or a step up of the spindle motor.
The passive rectification implies a rectification of the BEMF of the spindle motor through the intrinsic diodes of the power stage 12 which is operated at high impedance.
The synchronous rectification of the BEMF of the spindle motor takes place in an active manner through the sequential power up of two MOSFET transistors of the power stage 12 in synchronicity with the phase of the three backelectromotive forces of the coils L1-L3 of spindle motor 2.
The rectification by means of the spindle motor step up implies that the power stage 12 is continuously switched from a tristate condition to a braking condition at a frequency higher than 20 KHz (out of the audible range), instead of being kept under the tristate condition. Thereby, when the power stage 12 is under the braking condition (with all the low side transistors being switched on or all the high side transistor being switched on), the spindle motor 2 is under a short-circuit condition and therefore the three backelectromotive forces are able to generate a current in the motor. When the power stage 12 is driven in tristate, the three motor currents generated during the braking step recirculate through the intrinsic diodes of the six transistors of power stage 12, thus loading the capacitance C3 connected between the line where there is the voltage Vm and ground GND, keeping it at an enough potential so as to supply the power stage 13 and voice coil motor 3 for parking the reading and writing heads; said parking procedure begins when rectifying the BEMF of the spindle motor 2.
The parking procedure of the reading and writing heads may be commonly carried out either at constant voltage or constant speed.
In the case of constant-voltage parking, the voice coil motor 3 is driven by the stage 13 applying a constant voltage for a certain time period T1, preset with an appropriate polarity for moving the heads in the correct parking direction, or the voice coil motor 3 is driven by the power stage 13 applying a first constant voltage for a time period T1 and a second constant voltage higher than the first voltage for another time period T2.
In the case of constant-speed parking, the voice coil motor is driven so as to keep the speed of reading and writing heads controlled during the parking procedure. Various methods are known in the state of the art to keep under control the speed by which the voice coil motor takes the reading and writing heads to a parked position. This type of procedure ends when the heads reach the parking zone; the control circuit 1 also includes means such as a circuit or device adapted to detect when the reading and writing heads reach the parked position.
Once the reading and writing heads reach the parked position, the spindle motor 2 may be stopped; this generally occurs by short-circuiting the coils of the spindle motor through the activation of the low side transistors or high side transistors of the power stage 12 in a triple half bridge configuration. This procedure is commonly called “dynamic brake”. Thereby, with the spindle motor being short-circuited, the BEMF thereof generates a braking current, which is a function of the amplitude of the generated BEMF, and therefore of the instant speed of the motor and the impedance of the motor coils.
In specific applications, such as the hard disks for high end applications, the rotational target speed is very high; the speeds for these applications typically range from 10,000 to 15,000 rpm. To achieve this speed, the impedance of the motor coils is made very low; in such a case, if the motor was placed under a short-circuit condition immediately after the parking procedure of the reading and writing heads, the current generated by the motor itself would have a value even higher than 5 or 6 amps.
In power combinations having the power stage with six transistors, such as the stage 12, such a current value very often exceeds the specification limits for the maximum current that may be driven by the power stage. For such a reason, due to reliability problems, at the end of the parking procedure the spindle motor braking may not be immediately activated through the power up of the low side transistors of the power stage 12, since the high current would damage the power stage 12. It is therefore required to wait for the speed of the spindle motor 2 to become lower than a predetermined speed Vf such that the generated BEMF then forces a lower current to the maximum level that may be controlled by the power stage when the spindle motor is short-circuited to activate its braking. Therefore, in such a case, between the end of the heads parking step and the spindle braking activation there is a waiting time Ta in which it is controlled (by known means) when the speed of the spindle motor 2 becomes lower than the value Vf, so that the short-circuit of the coils thereof may be activated without the BEMF-generated current exceeding the specification limits of the power stage.
This waiting time Ta may also be very long. In fact, spindle motors for high end applications are characterized in that they spin at high speeds and that they have a very high moment of inertia; the waiting time for the speed to decrease from target (e.g., 15,000 rpm) to the speed Vf of activation of motor coil short-circuiting (e.g., lower than 8,000 rpm) is very long (higher than 10 seconds).